This invention relates to medical diagnostic ultrasonic imaging, and in particular to methods and apparatus for reducing sampling rate requirements in ultrasonic imaging and for improving fundamental/harmonic separation.
Modern ultrasound imaging systems using digital techniques for receive beamformation are limited in their frequency coverage by practical sample rates of analog-to-digital converters for reception. Standard practice involves sampling rates at or near four times the system carrier frequency; this permits complex baseband demodulation in the digital domain using simple, low-distortion techniques. A system with, for example, 40 MHz sampling is thus practically limited to 10 MHz imaging.
One object of certain embodiments of the present invention is to increase the useful imaging frequency, relative to the sampling frequency, by a factor of two. In the example above, a system capable of 40 MHz sampling can be extended to 20 MHz imaging.
Harmonic imaging is an increasingly important imaging mode for ultrasound imaging systems. In harmonic imaging the target is insonified with ultrasonic energy in a fundamental frequency band, and echoes from the target at a harmonic of the fundamental frequency band are imaged. In such systems it becomes important to suppress echo signals in the fundamental frequency band, and in the past this has been accomplished with the use of filters and with two-pulse cancellation techniques.
Another object of other embodiments of the present invention is to improve the suppression of either fundamental or harmonic echo signals in an ultrasonic imaging system.
This invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims.
By way of introduction, the methods and apparatus described below acquire a complex signal as a function of a backscattered signals from plurality of transmit beams. These transmit beams are created using separate respective components (such as the real part and the imaginary part) of a desired complex insonification signal, and the corresponding echo signals are combined in various ways to obtain important advantages. As explained below, the echo signals can be combined to emphasize linear echoes and to allow the sampling rate for backscattered echo signals to be made as low as two times the central carrier frequency fO of the backscattered signals. The backscattered echo signals can be combined in other ways to selectively enhance either the fundamental or the harmonic components of backscattered signals that include non-linear components.